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1. Are you certain that both the Mac and the iPod touch are authorized on the same iTunes account?

2. If you right-click on the iPod touch (in the iTunes source column), can you choose "sync back" or something like that? And if you do, does it copy over the songs to the Mac?
 
This happened to me and this is how I fixed it.

In the iTunes Store (the one on the computer) sign into your account and view your account (do this by clicking the button which says your username at top right corner)

Then go into purchase history and click "Report a Problem" on the tunes which you bought on your iPod Touch. When sending it to apple state that it got deleted when you synced.

In summary, basically tell :apple:'s iTunes Support!
 
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